Concrete structures are commonly reinforced with embedded steel cables referred to as tendons. Their presence serves to enhance the load supporting capabilities of the structures and to limit sagging. This is done by forming the structures with embedding tubes through which the tendons loosely extend. Once the structure has been erected the tendon can be anchored to one end of the structure and the other end then elongated out of the opposite end and anchored while being stressed.
The magnitude of stress that should be imparted to a tendon can be calculated for structures of given sizes and designed load capacities. With this known a tendon stressing jack can be used to stress the tendon to the calculated degree of stress by reference to a jack hydraulic pressure gauge. In other words, by referencing the pressure exerted by the hydraulic jack one may theoretically determine the stress on the tendon as it is being stressed. Thus, once a proper reading is made the tendon is permanently anchored in that condition of stress.
It sometimes occurs that calculations of tendon stress derived from jack hydraulic pressure readings are erroneous. For example, sometimes the tube through which a tendon passes becomes breached thereby enabling concrete to enter and directly embed a portion of the tendon. In addition, tendons may become snagged from other causes. When this occurs jack operations fail to stress that portion of the tendon located beyond the embodiment or snag which condition is not made apparent from hydraulic pressure gauge readings of the jack. Because of this tendons are often marked with paint to measure elongation as a safety crosscheck. This however is relatively crude and inaccurate since the markings tend to be applied differently by different workers and become blurred and faint from grease and handling.
Accordingly, it is seen that if a tendon stressing jack could be devised that could measure the stress being applied to a tendon during jack operations in an accurate, reliable and easy manner, a distinct advance in the art would be achieved. It is to the provision of such that the present invention is primarily directed.